1. Technical Field
The present invention relates to a capacitive type sensor, and more particularly, to a capacitive type sensor capable of suppressing sensor's drift deterioration caused when the sensor is placed in a high temperature/humidity environment.
2. Related Art
Humidity sensors having a water vapor sensitive film of polymer material are generally classified into capacitive type sensors and resistive type sensors. Capacitive type humidity sensors are advantageous in that they have an excellent response to environment's humidity variation, a low output hysteresis with increasing/decreasing environment's humidity, and a less humidity-dependent water vapor sensitivity, and can make a measurement in a wide humidity/temperature range.
A parallel plate capacitive type humidity sensor, which is mainstream in capacitive type humidity sensors, generally comprises an insulating substrate; a lower electrode formed on the substrate; a water vapor sensitive film formed on the lower electrode and made of an electrically insulative, water vapor sensitive polymer material; and a porous upper electrode formed on the film. Thus, the lower electrode, water vapor sensitive film, and upper electrode are formed in layer in this order on the substrate. Lead wires are connected to the lower and upper electrodes, respectively.
When this humidity sensor is placed in an environment, water vapor (gas), i.e., water molecule, passes through the porous upper electrode to reach the inside of the water vapor sensitive film, to be absorbed therein. The water vapor absorption continues until the equilibrium of water vapor is established between the environment and the film. At the equilibrium, there occur water vapor absorption and desorption on the film, so that the resultant amount of water vapor in the film corresponds to the environment's relative humidity. On the other hand, the sensor's capacitance varies in proportion to an amount of water vapor absorbed in the film. Therefore, the environment's relative humidity can be measured based on the sensor's capacitance. To carry out a humidity measurement, an electrical output signal of the humidity sensor indicative of the sensor's capacitance is supplied to a humidity measuring device.
Prior to humidity measurement, the sensor's water vapor sensitive property is stored in the humidity measuring device. To this end, standard environments are prepared whose temperature is at 25 degree centigrade and whose relative humidities are individually at x0, x1, - - - , xn, for instance. With the sensor placed in the standard environments in sequence, sensor's capacitances c0, c1, - - - , cn are measured by an impedance analyzer or the like. Then, a relationship between sensor's capacitance and environment's relative humidity, i.e., the sensor's water vapor sensitive property, is determined and stored in the humidity measuring device.
In an actual humidity measurement, the humidity measuring device receives an output signal from the humidity sensor placed in an environment whose humidity is to be measured. On the basis of the sensor output signal representative of the sensor's capacitance and the sensor's water vapor sensitive property representative of the relationship between capacitance and relative humidity, the humidity measuring device detects a relative humidity of the environment. For example, when the sensor output signal represents a capacitance cn, a relative humidity xn is detected.
However, the parallel plate capacitive type sensor placed in a high temperature and humidity environment for long time tends to generate an electrical output signal representing a relative humidity higher than an actual humidity of the environment, resulting in an error in humidity measurement. For example, even if an actual relative humidity is at xn, the sensor generates an inaccurate output signal cn+Δc, so that an inaccurate relative humidity xn+Δx is detected based on the output signal cn+Δc. Such phenomenon of an inaccurate output signal, representing a relative humidity higher than an actual relative humidity, being generated from a sensor placed in a high temperature and humidity environment is usually called as sensor's drift deterioration. According to the understanding of the present inventors, the drift deterioration is caused by a large swelling of the water vapor sensitive film of the sensor placed in a high temperature and humidity environment, as described later in detail.
To improve the measurement reliability of a parallel plate capacitive type sensor, the water vapor sensitive film is required to have an increased area size. Thus, the resultant sensor is large in size, failing to meet the demand of making the sensor compact.
This sensor is applicable to measurements for polarized formaldehyde, polarized acetone, polarized alcohol, etc., however, it entails problems of drift deterioration and upsizing also in that case.